Recently, the IEEE 802.11 wireless LAN has been widely used for wireless Internet access because the IEEE 802.11 wireless LAN can be installed through a simple method and maintained at a low cost. Currently, the IEEE 802.11 wireless LAN defines two different media access control schemes.
As a basic access scheme, the IEEE 802.11 wireless LAN uses a distributed coordination function (hereafter, referred to as “DCF”). The DCF is a general carrier sense multiple access scheme with collision avoidance (CSMA/CA) which can be used in both of an ad-hoc network and an infrastructured network environment.
Furthermore, the IEEE 802.11 wireless LAN may selectively use a media access control scheme referred to as a polling-based point coordinate function (PCF).
A wireless network has a characteristic of broadcasting. When two stations use a channel having the same band, the two stations cannot transmit data at the same time. When two or more stations transmit data at the same time, a collision may occur. In this case, all of the transmissions may fail.
The conventional DCF includes three inter-frame spaces (IFS), that is, a short inter-frame space (SIFS), a PCF inter-frame space (PIFS), and a DCF inter-frame space (DIFS), in order to implement the IEEE 802.11 media access control scheme.
The IFS indicates a minimum waiting time which is necessarily required before a next frame transmission operation is performed after one frame is transmitted. The three inter-frame spaces have a relationship of SIFS<PIFS<DIFS. Since the SIFS is the shortest, the SIFS is used for a communication having the highest priority.
Hereafter, a general method for stochastically avoiding a collision occurring in a wireless environment using a DCF will be described.
The DCF uses a binary exponential back-off scheme, in order to avoid a collision between transmit (Tx) stations and to prevent a specific station from consecutively occupying a channel. The binary exponential back-off scheme reduces the probability of consecutive collisions by setting a delay time until a transmission is attempted again, when data transmission signals on a transmission medium collide with each other in a wireless LAN.
Before data are transmitted, all stations check whether the transmission medium is occupied by another station. At this time, when the transmission medium is not used during a DIFS, each of the stations considers that the transmission medium is idle, and immediately transmits data. A station receiving the data transmits an ACK frame immediately after an SIFS, the ACK frame indicating that the data was successfully transmitted. The station having succeeded in transmitting data preferentially selects a back-off number as an arbitrary value smaller than a contention window, regardless of whether the station still has data to transmit, and starts back-off.
Since the SIFS is smaller than the DIFS, a transmission of another station does not interfere with a transmission of an ACK frame. When no ACK frame is received, the station considers that the transmission failed, and doubles the value of the contention window. The initial value of the contention window is set to CWmin, and the maximum value of the contention window is set to CWmax. When a transmission is successfully performed, the contention window is reset to CWmin.
When the transmission medium is occupied by another station, that is, when the transmission medium is busy, the corresponding station selects the back-off number to an arbitrary value smaller than the contention window. At this time, when the transmission medium is not used during a DIFS, the station considers that the transmission medium is idle, and starts back-off.
When a transmission of another station is not performed during a predetermined slot time, the station reduces the back-off number one by one. Then, when the back-off number becomes zero, the station transmits data. When a transmission of another station is recognized, the station stops back-off. Then, when the transmission medium becomes idle again during a DIFS, the station resumes back-off.
Then, when the back-off number becomes zero and the transmission medium is idle, the station starts transmission.
Although the conventional DCF was adopted as a basic media access control scheme of the IEEE 802.11 wireless LAN, the conventional DCF exhibits low performance. Furthermore, as the number of stations increases, collisions may frequently occur, and a bandwidth may be wasted by unused back-off slots. Thus, the conventional DCF has a low transmission rate.
In order to solve the above-described problems, a variety of schemes have been proposed. However, in a general competition-based distributed media access control scheme, collisions and unused back-off slots have a trade-off relationship. Thus, when one is intended to be reduced, the other is increased. Therefore, such an access scheme has a limitation in improving performance.